1. Iron(III) sulfate is an ionic compound. Cation is iron(III) ion. Fe3+
QUESTION:
Which of the following is correct formula for iron(III) sulfate? A. Fe3S, B. FeSO4, C. Fe2(SO3)3 , D. Fe2(SO4)3
Iron(III) sulfate is an ionic compound.
Cation is iron(III) ion. Fe3+
Anion is sulfate: SO42-
A compound is electrically neutral. The formula for an ionic compound should reflect the simplest set of ions with a total charge of zero. Choice B. Fe3+, SO42-; total positive charge = +1
Choice D. Fe3+, Fe3+, SO42-, SO42-, SO42-, total charge=0
FeSO4 is iron(II) sulfate.
SCRIPT:
Which of the following is correct formula for iron(III) sulfate? A. Fe3S, B. FeSO4, C. Fe2(SO3)3 , D. Fe2(SO4)3
PAUSE
To determine the formula of a compound given its name, we need to first classify the compound, then apply the rule for that type of compound.
CLICK
Iron three sulfate is an ionic compound….
You should know that if the name of a compound starts with the name of a metallic element it’s ionic.
Cations are generally derived from atoms of metallic elements, such as iron.
CLICK A roman numeral three written next to the name of a transition or post-transition metal generally tells us the charge of the cation. So, in this case, we know that the cation is iron with a +3 charge.
CLICK The first part of the name of any ionic compound is the name of the cation.
The rest of the name of the name of the anion.
In this case, it’s sulfate.
And the formula of sulfate is SO4 with a –2 charge.
HIGHLIGHT SO42-
How do we know this? It’s trivial. We look it up. Or better yet, we memorize it.
At this point, we can eliminate choices A and C.
CROSS OUT S in choice A, 3 in SO3 in choice C.
There is no SO4 grouping either of these choices.
So, how do we decide between choices B and D? CLICK A compound is electrically neutral. The formula for an ionic compound should reflect the simplest set of ions with a total charge of zero.
Choice B would imply a total charge of +1. The formula in choice B implies a formula unit consisting of…
one iron(III) ion, which has a charge of +3.
HIGHLIGHT Fe3+
And one sulfate ion, which has a charge of –2.
The sum of +3 and –2 is +1, which isn’t right.
How about choice D?
CLICK The formula implies that there are …
Two cations
HIGHLIGHT Fe3+, Fe3+,
And three anions
HIGHLIGHT SO42-, SO42-, SO42-,
In one formula unit.
HIGHLIGHT charges as numbers are called
+3… plus +3… plus –2… plus –2… plus –2, equals zero.
Therefore, choice D is the correct formula for iron(III) sulfate.
Choice B is, actually, a correct formula. But is the correct formula for iron two sulfate.
The cation in this case has a charge of +2, and the anion has a charge of –2.
+2 plus –2 equals zero.
CLICK PAUSE END RECORDING

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08